This invention relates generally to the field of materials technology, and more specifically to the field of thermal barrier coatings for high temperature applications, and specifically to a process for manufacturing a turbine component by applying layers of a thermal barrier coating using a cold spray technique, and to a component manufactured with such a process.
It is well known that the power and efficiency of operation of a gas turbine engine or a combined cycle power plant incorporating such a gas turbine engine may be increased by increasing the firing temperature in the combustion portions of the turbine. The demand for improved performance has resulted in advanced turbine designs wherein the peak combustion temperature may reach 1,400 degrees C. or more. Special materials are needed for components exposed to such temperatures. Nickel and cobalt based super alloy materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades. However, even super alloy materials are not capable of surviving long term operation in a modern gas turbine without some form of insulation from the operating environment.
It is known to coat a superalloy metal component with an insulating ceramic material to improve its ability to survive high operating temperatures in a combustion turbine environment. One thermal barrier coating system 10 in common use today is illustrated in FIG. 1. A ceramic top coat 12 applied to a super alloy substrate structure 18, with an intermediate metallic bond coat 16. An example of a commercially available super alloy material 18 is IN738 made by Inco Alloys International, Inc. A common ceramic insulating material 12 is yttria stabilized zirconia (YSZ). Hafnia or scandia stabilized zirconia may also be used as layer 12, or alternatively, yttrium aluminum garnet (YAG). The bond coat layer 16 provides oxidation resistance and improved adhesion for the thermal barrier coating layer 12. Common bond coat materials 16 include MCrAlY and MCrAlRe, where M may be nickel, cobalt, iron or a mixture thereof. The metallic bond coat material 16 has the additional function of supplying aluminum to form a thermally grown oxide (TGO) layer 14, which may be formed substantially of aluminum oxide. The oxide layer 14 develops during manufacturing heat treatment operations and during the operating service of the turbine, and it may grow from 0 to 15 micrometers thick through the life of the coating 10. This oxide layer provides oxidation resistance for the underlying super alloy 18 and provides an improved bond between the ceramic layer 12 and the metallic bond coat 16. The thermally grown oxide layer may alternatively be grown on a platinum enriched bond coat or platinum aluminide. To achieve such an embodiment, a layer of platinum is first applied to the surface of the bond coat layer 16 and then is diffused into the bond coat layer 16 by a diffusion heat treatment.
The metallic bond coat layer 16 is known to be applied by any one of several thermal spray processes, including low pressure plasma spray (LPPS), air plasma spray (APS) and high velocity oxy-fuel (HVOF). Such processes propel the MCrAlY material in a molten plasma state against the surface of the super alloy substrate 18 where it cools and solidifies to form a coating. Such thermal spray process are known to result in a significant amount of porosity and the formation of oxygen stringers in the bond coat layer 16 due to the inherent nature of the high temperature process. The release of heat from the molten particles of MCrAlY and the transfer of heat from the high temperature gas used in the thermal spray process also result in a significant increase in the surface temperature of the super alloy substrate material 18 during the metallic bond coat 16 application process. Such elevated temperatures result in localized stresses in the super alloy material 18 upon the cooling of the coating layer. Furthermore, a post-deposition diffusion heat treatment is necessary to provide the required metallurgical bond strength, and such treatment may have adverse affects on the material properties of the underlying substrate.
The known processes for manufacturing thermal barrier coating systems have numerous limitations, such as the creation of residual stresses, the formation of coating layers containing voids and porosity, the need for specialized thermal spraying equipment that is not adaptable for field repair operations, and a high cost of manufacturing. Thus, an improved process is needed for manufacturing components having a thermal barrier coating.
The present inventors have recognized that a cold spray process is beneficial for the application of a metallic bond coat layer of a thermal barrier coating for a combustion turbine engine part. The cold spraying of bond coat powders allows for the deposition of a dense oxidation and corrosion resistant coating on both new and service-run gas turbine components. Because a cold spray process produces a coating having essentially no porosity and no oxygen stringers, the performance of the bond coating during the operation of the component will be improved when compared to prior art flame or thermally sprayed coatings.
Because the area to which a coating is applied may be limited and controlled during a cold spraying process, new components may be fabricated using a cold spray process for filling gaps or discontinuities in a bond coat layer during the original manufacturing process. Furthermore, components that have been damaged by mishandling or by out-of-specification machining may be repaired using a cold spray technique.
In one embodiment of the invention, the thickness of a bond coat layer is varied along a surface of a turbine component, with a thicker coating being applied in those areas of the component exposed to the highest temperatures during turbine operation. In a further embodiment of the invention, the composition of a bond coat layer is varied along a surface of a turbine component. This may be particularly advantageous to reduce the consumption of an expensive material, such as platinum, by limiting the application of such material to only those portions of the component where the resulting benefit is necessary. Such variations in coating applications may be accomplished without masking, thereby eliminating process steps and eliminating the geometric discontinuity normally associated with the edge of a masked area.
In a further embodiment of the present invention, the composition of a bond coat layer applied by a cold spray process is varied along a depth dimension. The material of the first layers to be applied is selected to minimize interdiffusion with the underlying substrate material, and the material of the top layers is selected to optimize resistance to oxidation and corrosion.
A process for manufacturing a turbine component in accordance with the present invention is more economical than prior art processes because there is no need for a high temperature heat treatment following the deposition of the bond coat. As a result, the initial interdiffusion zone between the substrate and bond coat is minimized. Thus, the amount of aluminum available in the bond coat layer for the subsequent formation of a thermally grown oxide layer is increased when compared to components formed with prior art processes. This results in improved performance of the thermal barrier coating system.
In a further embodiment of this invention, the roughness of the surface of a bond coat layer is controlled to a desired value by controlling the parameters of a cold spray process. A desirable degree of roughness may be obtained without the need for post-deposition processing.
These and other objects and advantages of the invention are provided by way of example, not limitation, and are described more fully below.